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objectstate exports a function that constructs a stream. The stream is designed to be the source of truth for the state of a system. It represents state as a JavaScript object, and updates it by 'listening' to streams or event emitters.

When any stream or event emitter emits new values, ObjectState updates its state representation, and fires a "data" event. Client code can subscribe to the "data" event for a full representation of the state of the system each time it changes.


var EE = require('events').EventEmitter
var objectState = require('objectstate')
var through = require('through')
var stream = through()
var ee1 = new EE
var ee2 = new EE
var os = objectState()
os.listen(stream, 'rat')
  .listenOn(ee1, 'data', ['cat', 'dog'])
  .listenOn(ee2, 'error', ['hat'])
os.on('data', function (state) {
stream.queue(5)             // {"rat": 5}
ee1.emit('data', 1)         // {"rat": 5, "cat":1}
ee1.emit('data', 1, 2)      // {"rat": 5, "cat":1, "dog":2}
ee2.emit('data', 100)       // does not log, since os does not listen to ee2's
                            // data event.
stream.queue(5)             // does not log, because this changes nothing.
ee2.emit('error', "hello")  // {"rat": 5, "cat":1, "dog":2, "hat":"hello"}
ee1.emit('data')            // {"rat": 5, "hat":"hello"}


objectState(_initial) -> DuplexStream

  • _initial is an optional object to use as the initial state. If a non-object is provided, a TypeError will be thrown.

ObjectState instances are readable/writable streams. ObjectState emits whenever its internal state changes, and can be written to with an object to set its internal state. If a non-object is written, an error event will be emitted. Writes of undefined are ignored entirely.


ObjectState will never alter any object that is passed to it, instead it makes a deep copy for use internally. Likewise, it only ever emits a deep copy of its state in order to avoid outside mutation.

For performance reasons, deep copy is implemented using JSON.parse(JSON.stringify(state)), which has a few limitations.

  1. It will throw an error if there is a self-reference,
  2. It will have strange side effects on properties that are not null, string, boolean, array, or a plain JavaScript object. This includes functions, typed arrays, and objects with prototype other than Object.prototype.

In practice, these limitations are mostly inconsequential. ObjectState is meant to manage data, not complicated instances.

ObjectState only ever emits when its internal state changes, a condition determined via a deep comparison of the new state object versus the previous one.

ObjectState speaks "keypaths", meaning you can reference deeply nested properties by dot-separated strings. For example, given the object {animals: {cats: {sound: 'meow'}}}, 'animals.cats.sound' would refer to the string 'meow'. If you do not need deep property access, a regular string will work as you would expect.

Instance Methods

os.listen(stream, keypath) -> os

  • stream: A stream
  • keypath: The keypath to update on emit

os.listenOn(ee, eventName, keypaths) -> os

  • ee: An event emitter
  • eventName: When ee emits events named eventName, their objects are recorded on the internal state object.
  • keypaths: An array of keypaths.

When the ee emits the event eventName, ObjectState saves each argument passed to the event handler under the keypaths array element at the same index. The Nth argument passed to the listener is saved under the Nth element of keypaths: state[keypaths[N]] = arguments[N].

If an emitted argument is undefined, ObjectState deletes the corresponding keypath in its internal state object.

If a specified parameter is falsey, it is skipped over during assignment.


Returns the value for state keypath keypath (or undefined if not set).

os.set(keypath, value)

Set the value for keypath keypath to value on the state.


Delete the keypath keypath from the state.


Emit the current state as a data event.


fn is a function that is immediately called. All changes that happen during its execution will be collected into a single data event, if the state changed at all.


Returns a deep copy of the current state object.


Please see CONTRIBUTING for details on how to contribute to this project.


This project is licensed under the Apache License, Version 2.0. See LICENSE for the full license.

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